1. Field of the Invention
This invention relates to heating, ventilation, and air conditioning, and more particularly to a system for maintaining good air quality in the spaces being conditioned.
2. Background of the Invention
Environmental comfort has recently become an increasingly important concern for modern office buildings. In response, many designers are now offering heating, ventilation, and air conditioning (HVAC) equipment that not only maintains a more uniform distribution of temperatures, but also allows individual occupants to better control the comfort of their individual spaces. An additional goal of these modern systems is to improve the overall office environment by delivering properly humidified, good quality air while simultaneously maintaining a suitable sound level at full and part load.
An office building floor may be conceptualized as a series of thermal zones. Thermal zones result from varying loads acting in different regions of the building. For example, the outermost zones experience "skin load" which results from effects such as thermal transmission through an outer wall or window, air infiltration from outside, and/or solar heating. Interior zones are not as susceptible to these loads, but will be affected by heat given off from people, lights, office equipment, etc. Perimeter zones, for example, sizeable exterior offices, are affected by all of these loads. In a simple open plan space, the perimeter zone is usually considered to extend inwardly from the outer walls of the building to a distance approximately 1.5 times the ceiling height.
A simple, rectangular building normally has a minimum of nine zones per floor, but many larger buildings can have more than twenty zones per floor. The loads required to maintain the temperatures of the zones can vary depending on the time of day or how they are used. For example, the motion of the sun causes significant load shift in buildings, especially those buildings with a high percentage of windows. Proper zoning enables the environmental control system to maintain comfort in all of the zones, even when the loads shift.
In a modern indoor environmental quality air handling system, primary air, typically outdoor air mixed with an amount of return air, is filtered, humidified or dehumidified, and heated or cooled to a temperature typically between 40-50.degree. F. (4-10.degree. C.) at a central unit. A central fan unit blows the treated primary air into supply ducts which circulate the primary air throughout the floor, or ceiling, or other air containment channels in the building to the various zones, each zone usually containing a number of terminal units. Each terminal unit serves an individual space, such as an individual office, or group of spaces and is controlled in response to space or zone thermostats.
Most modern office buildings use variable-air-volume (VAV) HVAC systems. Generally, there are five types of VAV terminal units available: dual duct, cooling only, cooling with reheat, series fan terminals, and parallel fan terminals. Cooling only and cooling with reheat units typically treat (heat/cool/humidify/filter) all air delivered to a space at a primary (central) air handling unit and do not utilize air from the ceiling plenum or other spaces to increase air flow or temper the air being delivered to the space without passing that air through the primary air handling units.
Fan terminal boxes and dual duct/dual fan systems utilize a method of diluting contaminant concentrations in a given space by increasing the flow of return or recirculated air from other spaces (secondary air) to the given space. A series fan terminal has a small secondary air fan which conveys primary and/or secondary air to the space that it serves. This fan delivers essentially a constant volume of air at all times. At the inlet of the unit, is a primary air inlet that is connected to the supply duct through a primary intake. A damper is provided in this primary air inlet, and is normally modulated on command of a space thermostat to vary the amount of treated or primary air to maintain space temperature. This series fan unit also has a secondary intake which draws secondary, or recirculated air from the ceiling area, or plenum, in which the unit is mounted. The secondary intake may also be connected to a return duct which channels air from the other spaces.
A parallel fan unit also has an inlet to draw secondary air from the ceiling plenum or return air ducts and a primary inlet with a damper that receives cooled or heated air (primary air) from the primary supply duct. Unlike the series fan unit, however, the primary air in a parallel terminal unit bypasses the secondary fan and discharges directly through an outlet into the space being served. The fan in the terminal unit is positioned in the secondary air inlet and draws secondary air and discharges it to the space. Thus, the fan can blow secondary air "in parallel" with primary air into the space.
A VAV terminal unit generally operates in the following manner. In a cooling operation the primary air is usually cooler than the desired temperature of the occupied spaces. Therefore, a signal from the thermostat that the temperature in the space is below the desired temperature will prompt a controller to close the primary air damper. The secondary fan in a series or parallel unit will compensate for the reduction of primary air by drawing in relatively more air through the secondary air inlet, thus providing a relatively constant volume of air to the space. The secondary air is already relatively warm because it is being drawn from occupied spaces and/or has picked up additional heat from lights, office equipment, body heat, etc. These units may also include an auxiliary heater to further heat the recycled air if the recycled air is not warm enough. In a series terminal, the secondary fan must run whenever air for heating, ventilating, or air conditioning is required in the space because air delivered to the space, whether it's primary cold air, or recycled warm air, passes through the terminal unit fan. The fan in a parallel terminal is run only when secondary flow is required.
If the space temperature is above the set point of the space thermostat, the damper in the primary air duct is controlled to deliver a volume of cool air to the space. In a parallel terminal unit, the secondary fan turns off when the primary damper opens beyond a set level. As the space temperature falls below the desired temperature, the primary air damper begins to close and delivers less cool air to the space. When the primary airflow is reduced, the amount of secondary air blown through the box is increased to deliver a net constant volume of total air to the space to keep the air circulation at an acceptable level. When the room or building is unoccupied, the controller may at times close the primary air damper and cycle the terminal unit fan (and auxiliary heat) on and off as necessary to maintain the desired "unoccupied" temperature in the space. In addition, a controller for one or more terminal boxes may send a signal to a controller on the central air handling unit to turn on and supply cold primary air if the space temperature rises above the "unoccupied" cooling set point.
Dual duct dual fan VAV systems are similar to parallel fan terminal units in that primary air, a mixture of outdoor air and return air or 100% outdoor air, is delivered to a VAV terminal near the space to provide a desired air flow to the space on demand from the space thermostat. Instead of individual fan terminals drawing air from the return plenum to provide increased air flow and or heating capability to the space, the return air from the spaces is returned to a common "hot duct" or "neutral duct" fan where the air is heated and/or filtered, if required during the winter months, or recirculated as either neutral or cooled air during the summer months. This air is delivered to the space either through a separate VAV box also under control of the space thermostat, or to a "dual-duct terminal unit" that has two inlets and two damper assemblies both under control of the space thermostat.
If the space temperature is above the set point of the space thermostat, the damper in the primary cold air duct is controlled to deliver a volume of cool air to the space. As the space temperature falls below the desired set point, the primary cool air damper begins to close and delivers less cool air to the space. When primary cool airflow is reduced, the secondary (hot duct) airflow is increased to provide heat if necessary and keep room airflow at an acceptable level.
In the unoccupied mode, the primary cool fan is shut off, the primary cool air dampers in the spaces are closed, and the primary heating/neutral fan is cycled to maintain reduced space temperature. In some instances where the return/neutral fan handles cool air in the summer months, the damper opens to provide additional cooling if the primary cold duct cannot supply sufficient cool air.
In the past, energy efficiency has often overridden concerns for interior air quality. Air quality problems, however, can cause headaches, fatigue, congestion, blurred vision, and, in extreme cases, certain diseases. While the above systems and other conventional HVAC systems have been acceptable systems for heating and cooling spaces, they have not fully addressed the air quality of the interior spaces. A substantial but unresolved need has existed and does exist for HVAC systems that condition spaces efficiently, while still providing quality air which does not detrimentally affect the conditioned environments.
The difficulty of maintaining a uniformly acceptable air quality in all of the spaces served by a conventional environmental control system can be illustrated with reference to typical winter and summer operating conditions.
During the winter months, the perimeter spaces of the building require more heating than the interior spaces. When the controller determines that a space needs to be heated, it closes the primary air damper in the terminal unit. The secondary air damper then opens to introduce secondary air to the space. If the secondary air is not warm enough, it is heated by an auxiliary heater in the terminal unit. Most systems automatically close the primary air damper when the heated secondary air is blown through the unit to avoid wasting energy on simultaneous heating and cooling. In contrast, the intermediate spaces will not be subject to the temperature changes experienced at the perimeter, and are usually sufficiently warmed by body and equipment heat so that if any temperature regulation is required, it is usually to cool the spaces. Thus, cooling only terminal units are often located in the interior zones and deliver primary air to the interior spaces when the controller determines that the interior spaces are too warm. Therefore, most of the fresh, primary air will be delivered to the interior spaces because perimeter zones will so often need to be heated by the introduction of secondary air. This disparity of primary (outside) air distribution causes the concentration of CO.sub.2 in the perimeter zones to increase relative to the interior zones.
In the summer, the perimeter spaces are heated by thermal transmission through an outer wall or window, air infiltration from outside, and/or solar heating. In order to cool the perimeter spaces, the controller directs a greater amount of primary air to the perimeter spaces. The interior spaces, however, require relatively less cooling. Therefore, in the summer, the interior spaces receive relatively less primary air, and the air quality in the spaces suffers.
There have been some efforts to increase air quality and meet ventilation code requirements, but these efforts have lead to systems having limitations in efficiencies. For example, many systems deliver a constant volume of outdoor air, or at least more than a minimum amount of outdoor air, to the spaces at all times. The amount of outdoor air to be delivered is calculated based on the estimated maximum number of people in a space. Thus, in these systems, the amount of outdoor air delivered to the building is usually greater than the amount needed to meet appropriate standards. As a result, those system have considerable inefficiencies, particularly in the form of energy loss wasted to heat or cool the extra outdoor air.
Other systems monitor the amount of CO.sub.2 returning to the central unit through the return ducts, and decrease CO.sub.2 levels by introducing a greater amount of fresh, outdoor air to the central air treating unit. Thus, a certain amount of outdoor air is mixed with the primary air flow to dilute contaminants such as volatile organic compounds (VOC's), which are given off by building materials or certain office supplies. Because the CO.sub.2 level in the air returning to the central unit is monitored, these systems only control the total amount of CO.sub.2 in the building rather than the amount in the individual spaces. Therefore, in the summer excess outdoor air is delivered to the perimeter zones while the air quality of the interior suffers, and in the winter the perimeter air quality suffers while excess outdoor air is delivered to the interior zones even though the average CO.sub.2 concentration is at an acceptable level.
Delivering excessive outdoor air to a space is costly from an energy standpoint. First of all, energy is required to heat or cool, humidify or dehumidify, and filter the outdoor air at the central unit, and a central fan must be activated to deliver the primary air to the supply duct. Moreover, excess primary air requires heating by terminal units when it is delivered to the spaces. Therefore, energy efficiency is sacrificed by systems that simply provide excess outdoor air to decrease the CO.sub.2 level in the office spaces.